Pt Nanoparticle Sintering and Redispersion on a Heterogeneous Nanostructured Support
Journal article, 2016
Understanding how nanostructure and atomic-scale defects of the support affect metal catalyst nanoparticle sintering is of crucial importance to minimize thermal deactivation, as well as to understand the origin of widely observed but still unexplained phenomena, such as transient multimodal particle size distributions and nanoparticle redispersion. To shed light on these issues, we present a generic experimental approach that relies on nanofabrication to introduce controlled structural heterogeneity in a chemically homogeneous model catalyst support. This is achieved by fabricating arrays of nanocone structures separated by flat areas, where both are homogeneously sputter-coated with a thin amorphous alumina layer. Using ex situ aberration-corrected scanning transmission electron microscopy (STEM) to analyze Pt model catalyst nanoparticles on such nanostructured supports prior and after exposure to 4% O-2 in Ar carrier gas at 600 degrees C, we find that the initial particle size distributions and their time evolution during sintering to be different on the cones and the flat areas. On the cones, redispersion of Pt into highly abundant particles of about 1 nm occurs very rapidly. In contrast, particle shrinkage and growth combined with redispersion occur on the flat areas, leading to a broader and bimodal size distribution. These processes are amplified and efficiently demonstrated by the nanostructured surface because of (i) higher support defect density on the nanocones compared to the flat surfaces in between and (ii) initially different Pt particle size distributions on the cones and on the flat surfaces. Hence, the nanostructured surface facilitates the dear identification of catalyst redispersion in oxidizing conditions and experimentally identifies a mechanism that gives rise to (transient) bi- or multimodal particle size distributions during sintering.